Concepts in Condenser System (Vacuum Equipment)

Condenser System for vacuum creations and its types with design criteria

Condenser system and vacuum equipment are common to evaporators and vacuum pans. Nearly all these vessels use direct contact condensers, where the cooling water comes into direct contact with the vapour to be condensed.

Creation of Vacuum:

a) In  a condenser, we condense large volume of vapour  and it will only produce a comparatively small volume of water or (condensate) and this water runs out through the bottom of the condenser down the barometric leg with the waste water.

b) Therefore if we have condensed the large volume of vapor into small volume of  water in a condenser, the remaining volume or area must be a vacuum(reduced). Thus we have created a vacuum.

c) With the aid of an air pump or other, vacuum is produced in an enclosed vessel called a condenser, which communicates with the vessels to be maintained under vacuum.

d) Cold water  is  pumped  in to  the  condenser to  ensure  condensation of  vapour coming  from pan  or   multiple  effect  evaporator. The  Condenser  is  placed  at  a  height  that  the  water after  condensation  flow  out  by  gravity together  with  condensed  vapours. The Condenser is a barometric chamber extended at  bottom by a barometric  column dipping  into a well open to  atmosphere.

The Barometric Column :

There are two dimensions of importance in the barometric column

Height of the barometric column

Cross-section of the column

Height of the barometric column :

The height of the column represents the sum of 3 terms:

 Ho = head of water corresponding to the maximum vacuum in the condenser

h = head necessary to give the desired downward velocity of the water

s  = margin of safety as provision for abrupt variations in level of the water in the column.

H = Ho  + h + s

Ho  is  the  minimum  head  of water  required to have  maximum vacuum  in condenser. The height of the column should be of the order of 10 (instead of 76 cm): (Vacuum in mtr of Hg x Density of mercury/ Condenser inlet water density)

0.76 m X 13.6 = 10.33 m  where 13.6 = density of mercury relative to water.

h =The head  necessary  to  give  desired   velocity  of  water

h= (1 + a) ( V² / 2g)

h = head of water, in mtr necessary to maintain flow in the column, at velocity V

V = velocity of flow in the column, in m/sec

g = 9.8 m/sec²

a = coefficient,

The values of coefficient given by Hausbrand as follows

S  is  the  Safety  margin for  sudden  variations  in vacuum or flow in water. It may take  in general 0.50 mtr.

Cross-section of the column:

The cross-section of the barometric column requires simply to have sufficient cross-section to assure discharge of the water.

D = 

D = diameter of the column, in mtr

V = velocity of flow of water in the column, in m/sec

Q = weight of vapour to be condensed, in M³/sec

W = ratio of weight of injection water to weight of vapour condensed,

According to Peter rein More simply, following the form of equation

D > 1.116 x (Qw )^0.4

Qw = Total Water and vapour flow rate in condenser in M³/sec.

The  velocity  of  water in  the  barometric  column is  maintained at 2 to 3  mtr /sec. So that  the  air  bubbles  do  not  escape  and  rise  in  condenser. They should pass along with water through tail pipe. For single entry type condensers  it may take upto 4 to 5 mtr/sec.

Types of Condenser System :

From the extraction of the air point of view  the condensers can classified as

Wet air condensers : Condenser with combined vapour and air extraction.

Dry air condensers : Condenser with separate arrangement for air extraction.

In Sugar Industry generally used the following types of condensers

a) Barometric condenser

b) Multi-jet condenser

c) Single Entry Condenser

Barometric condenser

a) In Barometric condensers having two types. They are Co-Current barometric condenser and Counter Current barometric condenser.

 b) Co-Current, barometric condenser water and vapour are introduced to the condenser from top and the worm water is let out removed from the bottom.

c) Counter Current barometric condenser. water and vapor are introduced in opposite direction. Baffles with perforation are provided to ensure more contact time and surface between water and vapour

d) Compared to the co-current type, counter current type barometric condenser works more efficiently.

Height of the condenser:

The condenser is a device for exchange of heat between vapour and cold water. The exchange will be the more complete as contact between these two fluids is more intimate.

Height  of  Condenser is   decided  as  per  the  time  that  is  necessary for  the  water  to  remain  in  contact with  steam. While proper arrangement of baffles in condenser to increase the contact area than the  overall  height  of  3.65 mt ( 12 ft)  is  sufficient .

Cross-section of the condenser.

The cross-section will depend directly on the quantity of vapour to be condensed.

S = 0.16 m² / ton of  vapour  to  be  condensed  per  hour. where S = horizontal cross-section of the body of the condenser.

Condenser volume :

The  condenser  volume  of  0.75 m³/ 1000 kg  of  vapour  is  recommended  however  0.5 m³/ 1000 kg  vapours  can  be  considered  as  a reasonable  figure for rain  type  condenser. however, has determined experimentally that 0.6 m³/ 1000kg per hour was sufficient in most cases. In some designs provide maximum  effective contact area of water to vapour in that design it will take 0.4 to 0.3 m³/ 1000kg  per hour.

Also  can be cross checking by Volume  of condenser = Cross sectional area  x Height of the condenser

The cone at the bottom of the barrel should have a slope of 70° to the horizontal.

Diameter of cooling water pipe :

The cooling water may be supplied by a pump. Its effective head, h, will be equal to the geometrical head at the entry to the condenser, increased by the vacuum in head of water. We have then:

V = a x 

V = water velocity in the inlet pipe, in m/sec

a = coefficient depending on the length of the pipe, its bends, valves and other obstructions to flow. In general, ” a ” is of the order of  5

g = 9.8 m/sec²

h  = head of cold water, at entry to condenser, in mtr (h  = pressure Of water at inlet X head at mean sea level)

Multi Jet Condensers ( Wet air barometric condensers) :

Principle :

Jet condensers were first placed on the market by  Schutte – Koerting about 1930. They are based on the dynamic effect of jets of water which, penetrating into the body of water in the barometric column, enter with them, by friction, the air contained in the condenser. If the cross-section of the barometric column is small enough to ensure a suitable velocity, the bubbles of air do not rise into the condenser and are evacuated to the well at the foot of the column.

a) The main difference with the Multi Jet condenser is that, it does not require air pump, and although it requires more water than the dry air type condenser, it is more economical to operate and maintain.

b) This type of condenser has high pressure water jet nozzles fitted in the lower section of the condenser, directed straight at the outlet or barometric leg. The vapor inlet to the condenser is from the top of the condenser much above the level of the jet.

c) The nozzles are called as jet nozzles and they create vacuum in the system. The jet is given such manner, that the jet of water will flow exactly through the center of the tailpipe. On the top portion of the jet box another set of nozzles is fitted circumferential through which the flow of pressurized water is flowing towards the center of the box. This part of water is responsible for the condensation of vapour, which is called as spray nozzles.

d) As against the dry air type, the Multi Jet differs (wet type) in principle that due to the high velocity of water passing down the barometric leg, air or gas bubbles will not rise but instead it will be drawn away through the outlet with the waste water.

e) The pressure of water in both the parts to be employed as between 0.5 to 0.7 kg/cm2 for observing the pressure the gauges are fitted to inlet pipes. The proportion of the quantity of water is in ratio of 40% water to jet and 60% to spray.

Diameter of spray nozzle (Ds)

Ds = 

As (area of the each spray  nozzle ) = Qs / Vs

Qs = Quantity of water per each spray nozzle in M³/Sec = Quantity of water / No. of spray nozzles

Vs = Velocity of water at nozzle in m/sec = 

h = Pressure of water at spray nozzle in kg/cm² x head at mean sea level

Diameter of jet nozzle (Dj ):

Dj = 

Aj (area of the each jet  nozzle) = Qj / Vj

Qj = Quantity of water per each jet nozzle in M³/Sec = Quantity of water / No. of jet nozzles

Vj = Velocity of water at nozzle in m/sec = 

h = Pressure of water at jet nozzle in kg/cm² x head at mean sea level

Condenser Vapour pipe dia :

Quantity of vapour to condensed (Q) = Heating surface of the equipment x Evaporation rate.

Cross sectional area of the vapour pipe = Quantity of vapour in M³/sec / Velocity of vapour in m/sec.

Condenser Diameter :

Cross sectional area of the condenser =( Quantity of vapour in M³/sec + Quantity  of inlet water to the condenser in M³/sec) / Velocity of vapour in m/sec.

Single Entry Condenser:

a) Single  entry  condensers  are  having  only  one water  distributing box  for  spray  and  jet  nozzles. Out  of  total  water about  60 to 80 %  water  is  used  at  Spray  nozzles  and 20 to 40 %  water for  jet  nozzles.

b) Vapours  are  condensed  by  forming  fine  mist  inside  the  condenser, increasing  surface  area  of  contact  and  finally  minimum  requirement  of  water.

c) The  difference  between  approach  temperature ( difference  between  vapour  temp. and  condenser  water  tail pipe  temp) does  not  exceed  5 to 6⁰C.

d) The Specially designed high efficiency centrifugal spray nozzles fitted on the jet box create micro fine atomized spray, resulting into wider surface contact with  incoming vapours.

e) The difference between water outlet temperature and inlet temperature of 10°C is achieved. Under  automation the difference can achieved upto 15°C.

f) Single entry  and  Multi jet  condensers  are  of  Parallel  flow  type  wet  air  condenser. The Single  entry  condenser  requires  less water  than  conventional  Multijet  condenser due to the following reason.

g) In the single entry condenser design the spray nozzles are fitted in nozzle box which fitted at the center of the condenser.  Thus the spray water covers all the available area inside the condenser hence the spray water particles contact area for the vapour condensation is more than the multi-jet condenser system. Another impotent parameter in this design  tail  pipe  diameter  and  velocity  of  water  in  the  tail  pipe .

h) The nozzles dia calculation same as multijet condenser. The velocity of warm water (outlet water ) goes upto 4 to 5 m/sec. The design of bottom cone, its reducer and venture were placed important role.

Condenser Water Requirement

In a condenser the vapours entering, transfer heat to the cold injection water, the heat transmission depending on the temperature and quantity of water. Based on the heat balance the equation for arriving at the quantity of injection water is.

Cooling water ratio ( W )  i n Kg of injection water per Kg of vapour =   where

I = Total heat of vapour in Kcal/kg  ( For 55 ^oC temperature of vapour the total heat having 621 Kcal/kg)

For saturated online steam table purpose go through the link  Online Steam Table for Saturated Steam

Ti = temperature of injection water in °C

To = temperature of condenser outlet water in °C.

According to E.Hugot the optimum value of this  Ti and To difference is 10°C.

The difference in temperatures of vapour being condensed and the outlet water termed as approach should be minimum since the efficiency of the condenser operation depends on the fact that minimum quantity of water should absorb all the latent heat of vapour.

Under normal conditions the cooling water ratio to vapour condensed in around 50 to 60 for barometric condensers, 60 to 70 for Multi jet condensers and 40 to 50 for single entry type condensers.

In the next revision of this article will provide online calculation sheet of condenser design.

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